Method and apparatus for making electron discharge devices



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METHOD AND APPARATUS FOR MAKING ELECTRON DISCHARGE DEVICES Filed Dec.51, 1964 INVENTOR.

United States Patent Oil-ice 3,298,769 Patented Jan. 17, 1967 3,298,769METHQD AND APPARATUS FOR MAKING ELECTRON DISCHARGE DEVICES Charles L.Tourney, Danvers, Mass., assignor to Sylvania Electric Products, Inc., acorporation of Delaware Filed Dec. 31, 1964, Ser. No. 422,753 7 Claims.(Cl. 316-15) This invention rel-ates to the manufacture of electrondischarge devices, and particularly to discharge devices having anelectrode within an envelope containing gas, even in residualquantities, at a stage of manufacture when the electrode and itscoating, if any, are resistively heated to release gases therefrom. a

One example of such devices is the fluorescent lamp comprising a glassenvelope with an internal fluorescent coating, enclosing afillof-mercur-y and inert gas, and including spaced emissively coatedelectrodes for supporting an arc discharge by thermionicelectronemission and ion counterflow. In the process of manufacturing afluorescent lamp gaseous impurities are removed by exhausting atmospherefrom the lamp and flushing it with an inert gas, either simultaneouslyor sequentially. -Then, prior to adding the mercury and inert gas and.sealing the envelope, each electrode is processed by passing a currentthrough it to heat it resistively and dispel gases. Generallythese stepsare performed in many discharge devices whether mercury filled or not,or whether the electrode is coated or not. In fluorescent lamps theelectrode, prior to processing is coated with a mixture of alkalineearth carbonates which are subsequently decomposed to emissive oxides bythe above mentioned resistive heating with the release of carbondioxide. Also, gases entrapped in the electrode metal are driven out bythe heating. At the time the electrode is resistively heated otherresidual gases in various quantities will be present. And at the lowpressure of the exhaust they will have a determinableionizationpotential.

To bring the electrode to the desired temperature, typically in theorder of 1450 C'., it is necessary to pass current through the electrodeat a volt-age above the ionization potential of the residual gases.Hitherto the residual gases, including those dispelled from theelectrode, have ionized well below the voltage required to produce thedesired electrode temperature. When ionized, the residual gases form aconductive path shunting the current intended to heat the electrode andpreventing adequate outgassing of the electrode and its coating. Aninadequately outgassed electrode produces a lamp which is diflicult orimpossible to start, which quickly develops discoloration, and which hasa short life.

Thus, the object of the present invention is to provide a Way in whichan electrode of a discharge device may be resistively heated to adesired temperature despite the presence of ionizable gases.

According to the invention the process of making an electron dischargedevice having an electrode within an envelope containing gas comprisesheating the electrode by passing a current therethrough at a voltageabove the ionization potential of the gas, and simultaneously apply inga magnetic field to the electrode thereby to inhibit ionization of saidgas adjacent the electrode and shunting of electrode current throughionized gas.

Further according to the invention apparatus for outgassing theelectrode within the envelope of an electron discharge device containinga gas ionized at a predetermined potential comprises means to supplycurrent through said electrode at a voltage in excess of said potentialresistively to heat said electrode to outgassing temperature, and meansto apply to said electrode a magnetic field of strength to suppressionization of the gas and prevent shunting of said current throughionized gas.

For the purpose of illustration typical embodiments of the invention areshown in the drawing in which:

FIG. 1 is a schematic diagram showing one way of processing afluorescent lamp electrode;

FIG. 2 is a graph of electrode current versus electrode potential fordiiferent values of applied magnetic field; and

FIG. 3 is a schematic diagram showing another Way of processing anelectrode.

. Shown fragmentarily in FIG. lis a typical fluorescent lamp undermanufacture comprising a tubular envelope 1 whose inner surface 2carries a phosphor coating. At each end of the envelope 1 is anelectrode structure and mount including a glass stem 3 sealing the endof the envelope and having an exhaust tube 4 communicating through thestem with the interior of the envelope. Two

lead wires 6 extend through a press portion 7 of the stem. Across theinner ends of the leads wires 6 is a coated electrode 8 which may be ofthe coiled-coil or triple-coiled type.

At the stage of manufacture shown the lamp has been heated, flushed withinert gas and exhausted by a pump P to about 100 microns to 10millimeters pressure leaving some residual gases within the envelope. Atthis point the alkaline earth carbonates are decomposed to active oxidesby heating the electrode, which heating also drives off the producedcarbon dioxide. In fluorescent lamps, as well as other discharge deviceshaving uncoated electrodes, it is also highly advantageous to removegases en trapped in the wires forming the electrode 8. To heat theelectrode a voltage is applied from a suitable alternating currentsource E to the lead wires 6 causing the electrode to be heatedresistively by current through it.

To reduce the carbonate and to release entrapped gases the electrodemust be heated as high as 1450 C. To reach such temperatures voltagesWell in excess of operating voltage must be applied to the electrode todraw suflicient current.

. 2 at and above this voltage the residual gases in the envelope i-onizeand shunt current around the electrode so that further increase in thevoltage across and the current through the electrode is not possible.

According to the invention the shunting effect of the residual gases issuppressed by applying an external magnetic field B through the envelope1 and the electrode 8 at the same time that the heating current isapplied. As shown in FIG. 1 the field is produced by coils 9 wound oncores 11 and supplied by a battery E or any other suitable directcurrent source. Typically the field B of the magnet is in the order of500 to 2500 gauss, and many more turns 9 than are shown will berequired. A permanent magnet of such strength may be used instead of theelectromagnet shown in FIG. 1. And, as shown in FIG. 3, an electromagnetsupplied with alternating current may be used.

On FIG. 3 the primary T1 of a transformer is connected to alternatingcurrent lines A, C. One transformer secondary T2 is connected to thecoils 9a of an electromagnet wound on cores 11a. The filament 8 of thelamp to be processed is connected to a transformer secondary T3 suchthat current through the electrode is substantially in phase withcurrent through the electromagnet coils 9a.

By applying a magnetic field to the electrode 8 or 8:: at the same timethe electrode is heated resistively, the high voltages necessary to heatthe electrode to decom- For example, a typical VHO lamp electroderequires a current of 2.7 amperesv for proper posing and outgassingtemperature may be applied across the electrode, even though the voltageexceeds the normal ionization potential of the residual gases present,without causing the gases to ionize and shunt the electrode current.

In FIG. 2, two examples of relation between electrode voltage andcurrent and the external field are shown. Curve Bl shows the values ofelectrode current versus electrode current with an applied field of 1000gauss. With such a field a voltage of nearly 16, well above the normalionization potential of about 9 volts on curve B0, will produce anelectrode current of about 2.8 amperes, more than needed to produce thedesired processing temperature. With a field of 2500 gauss electrodevoltages over 18 and currents over 3.2 amperes may be obtained as shownby curve B2.

The values given are typical for a VHO lamp when a direct current fieldB is directed at the preferred angle of 45 to the axis 801 of theelectrode as shown in FIG. 1. However, the relative angle of the field Bmay be as shown in FIG. 3, or any other value between 0 and 90 with onlyslightly reduced effectiveness.

Thus it should be understood that the present invention is not limitedby the illustrative examples but comprises all modifications andequivalents falling within the scope of the appended claims.

I claim:

1. In the process of making an electron discharge device having anelectrode within an envelope containing gas, the steps comprisingheating the electrode by passing a current therethrough at a voltageabove the ionization potential of the gas, and simultaneously applying amagnetic field to the electrode thereby to inhibit ionization of saidgas adjacent the electrode and shunting of electrode current throughionized gas.

2. The process according to claim 1 wherein said current and magneticfield are alternating and in phase at the same frequency.

3. The process according to claim 1 wherein the magnetic axis of saidfield is at an angle of 45 to the axis of the electrode.

4. In the process of making an electron discharge device including anenvelope and an electrode therein for treatment above a predeterminedtemperature, the steps comprising exhausting the envelope to leave aresidue of gas normally ionizable at a predetermined potential, passinga current through the electrode at a voltage above said normalionization potential and effective to heat the electrode resistively tosaid predetermined temperature, and simultaneously applying to theelectrode an external magnetic field of strength to inhibit ionizationof said gas adjacent said electrode, thereby to prevent said gas fromshunting said current around the electrodes and preventing heating ofthe electrode to said predetermined treatment temperature.

5. In the process of making a fluorescent lamp including an envelope andan electrode therein with a coating decomposable to emissive state abovea predetermined temperature, the steps of exhausting the envelope toleave a residue of gas, passing a current through the electrode at avoltage above the normal ionization potential of the gas to heat theelectrode resistively to said predetermined temperature and therebydecompose said coating to emissive state, and simultaneously applying tothe electrode an external magnetic field of strength to inhibitionization of said gas adjacent the electrode, thereby to prevent saidgas from shunting said current and preventing heating of the electrodeto said predetermined temperature.

6. Apparatus for outgassing the electrode within the envelope of anelectron discharge device containing a gas ionizable at a predeterminedpotential, comprising means to supply current through said electrode ata voltage in excess of said potential resistively to heat said electrodeto outgassing temperature, and means to apply to said electrode amagnetic field of strength to suppress ionization of the gas and preventshunting of said current through ionized gas.

7. An apparatus for outgassing the electrode of a fluorescent lamphaving an envelope around the electrode, said electrode having a coatingdecomposable to emissive state above a predetermined temperature,comprising means to exhaust and maintain the envelope at a pressure ofresidual gas normally ionizable at a predetermined potential, anelectnomagnet for applying a magnetic field through the envelope to theelectrode, and a transformer having a primary for connection to analternating current source, a first secondary for supplying alternatingcurrent through the electrode at a voltage above the normal ionizationpotential of said residual gas, thereby to heat the electroderesistively to said predetermined temperature and decompose said coatingto emissive state, and said transformer having another secondary forsupplying alternating current to said electromagnet substantially inphase with the current through said electrode, whereby saidelectromagnet field inhibits ionization of said gas adjacent theelectrode and permits heating of the electrode to said predeterminedtemperature.

No references cited. RICHARD HQEANES, ]R., Primary Examiner.

4. IN THE PROCESS OF MAKING AN ELECTRON DISCHARGE DEVICE INCLUDING ANENVELOPE AND AN ELECTRODE THEREIN FOR TREATMENT ABOVE A PREDETERMINEDTEMPERATURE, THE STEPS COMPRISING EXHAUSTING THE ENVELOPE TO LEAVE ARESIDUE OF GAS NORMALLY IONIZABLE AT A PREDETERMINED POTENTIAL, PASSINGA CURRENT THROUGH THE ELECTRODE AT A VOLTAGE ABOVE SAID NORMALIONIZATION POTENTIAL AND EFFECTIVE TO HEAT THE ELECTRODE RESISTIVELY TOSAID PREDETERMINED TEMPERATURE, AND SIMULTANEOUSLY APPLYING TO THEELECTRODE AN EXTERNAL MAGNETIC FIELD OF STRENGTH TO INHIBIT IONIZATIONOF SAID GAS ADJACENT SAID ELECTRODE, THEREBY TO PREVENT SAID GAS FROMSHUNTING SAID CURRENT AROUND THE ELECTRODES AND PREVENTING HEATING OFTHE ELECTRODE TO SAID PREDETERMINED TREATMENT TEMPERATURE.